The present invention relates to a hydraulic brake system having an antiskid control system and a pressure fluid source, the pressure fluid of the source being adapted to be fed via a valve device and an antiskid control valve to a wheel brake cylinder, with the pressure fluid source including two pumps with a joint or common drive.
As it is known, hydraulic twin pumps serve to deliver pressure fluid to two separated circuits, with only one single drive being needed. Especially in the field of automotive vehicles, pumps are preferred that require a minimum mounting space. From German Patent DE-OS No. 2,436,321 the problem of having two pumps deliver pressure fluid to the same circuit and to switch one pump over to a depressurized circulation upon attainment of a predetermined pressure, while the other pump continues to operate, is known. Pumps of the aforementioned type are equipped with a joint drive, however, they require additional switching arrangements which will switch the feed cycle of a pump upon attainment of a predetermined pressure.
There is the difficulty in automotive vehicles with antiskid control systems of the hydraulic brake system having a comparatively high volume demand on pressure fluid during a control phase. If a feed pump is constructed according to a high volume demand and driven by the vehicle's motor, for example, this will result in a very large amount of power being taken from the vehicle motor, which is, however, not at all times required in the hydraulic brake system. Even if one pump in an arrangement having two pumps pumps pressure fluid in an idling manner upon attainment of a specific pressure level, as is discussed in the above-cited German Patent, there is still a comparatively great amount of power being used up unncessarily for the operation of the two pumps.
If the drive is constructed as an electric motor, a considerable reduction in the voltage of the main power supply will become obvious when the motor is switched on.